The purpose of this research is to test the hypothesis that certain bacteria in plaque at sites of periodontal health are associated with the risk of periodontitis at infected sites. Porphyromonas gingivalis, Prevotella intermedia, Treponema amylovorum, T. denticola, T. maltophilum, T. medium, T. pectinovorum, T. socranskii, T. vincentii, and any new species of Treponema isolated from study sites that develop periodontitis will be evaluated as risk factors for disease. A quantifiable PCR assay will be used to further define risk by associating numbers of target bacteria with disease. Specific Aim 1: Correlate infection of health-associated plaque by each of seven oral Treponema species, P. gingivalis, P. intermedia, and new Treponema isolates with development of periodontitis. These studies will determine which disease-associated bacteria infect health-associated plaque before clinical detection of periodontitis. Specific Aim 2: Isolate Treponema from new sites of periodontitis at study sites. One of the designated target species not previously evaluated, or a yet-to be discovered Treponema, may be an important risk factor for the transition from gingival health to periodontitis. Specific Aim 3: Use a quantifiable PCR to estimate numbers of study bacteria in plaque. Risk for disease may be associated with critical numbers of specific bacteria in health-associated plaque before periodontitis is detected. The long-term goal of this research is to develop a means by which sites of periodontal health at risk for disease can be identified and treated before deterioration occurs.